Page - 137 - in The Origin of Species

EFFECTS OF NATURAL SELECTION 137 of the many now existing low forms have not in the least advanced since the first dawn of life would be extremely rash ; for every naturalist who has dissected some of the be- ings now ranked as very low in the scale, must have been struck with their really wondrous and beautiful organisation. Nearly the same remarks are applicable if we look to the different grades of organisation within the same great group; for instance, in the vertebrata, to the co-existence of mam- mals and fish—amongst mammalia, to the co-existence of man and the ornithorhynchus—amongst fishes, to the co-existence of the shark and the lancelet (Amphioxus), which latter fish in the extreme simplicity of its structure approaches the in- vertebrate classes. But mammals and fish hardly come into competition with each other ; the advancement of the whole class of mammals, or of certain members in this class, to the highest grade would not lead to their taking the place of fishes. Physiologists believe that the brain must be bathed by warm blood to be highly active, and this requires aerial respiration ; so that warm-blooded mammals when inhabiting the water lie under a disadvantage in having to come con- tinually to the surface to breathe. With fishes, members of the shark family would not tend to supplant the lancelet; for the lancelet, as I hear from Fritz Miiller, has as sole com- panion and competitor on the barren sandy shore of South Brazil, an anomalous annelid. The three lowest orders of mammals, namely, marsupials, edentata, and rodents, co-exist in South America in the same region with numerous monkeys, and probably interfere little with each other. Although or- ganisation, on the whole, may have advanced and be still advancing throughout the world, yet the scale will always present many degrees of perfection; for the high advance- ment of certain whole classes, or of certain members of each class, does not at all necessarily lead to the extinction of those groups with which they do not enter into close competi- tion. In some cases, as we shall hereafter see, lowly or- ganised forms appear to have been preserved to the present day, from inhabiting confined or peculiar stations, where they have been subjected to less severe competition, and where their scanty numbers have retarded the chance of fav- orable variations arising.